Heretofore, as a polyepoxy composition containing a trisepoxide, there have been known an epoxide (confer JP-A-62-197415) of a polyphenol compound obtained by a condensation reaction of hydroxybenzaldehydes with phenols and an epoxide (confer JP-A-61-65876) of a polyphenol compound obtained by a condensation and addition reaction of unsaturated aliphatic aldehydes such as croton aldehyde with phenols. A cured product of an epoxy resin prepared from the polyepoxy compositions is excellent in thermal stability, however, it very readily absorbs water. As a result, it cause problems such as formation of cracks in a package when it is employed as an encapsulant for semiconductors, and a deterioration in insulating properties when it is employed as an electric insulator. On the other hand, there has been known a polyepoxy composition (confer JP-A-61- 123618) obtained from a polyphenol having alicyclic structures in the molecule which is derived from dicyclopentadiene to improve water resistance. The cured product of an epoxy resin prepared from the polyepoxy composition is excellent in water resistance, however, thermal stability and flame-retardability thereof deteriorate, thereby being not preferred in view of the reliability at high temperature.